1. Field of the Invention
The present invention relates to an image processor which performs image processing operations using a spatial filter.
2. Description of the Related Art
The prior-art image forming apparatus includes an image input unit embodied in a scanner, an image processor with a spatial filter and an image output unit. The image input unit reads an image to generate image data. The image processor processes the image data for improving the image quality, using the spatial filter. The image output unit outputs an image based on the processed image data. One of the image processings using the spatial filter is removal of moire produced in a dotted image. Another image processing using the spatial filter is emphasis of edges present in the image.
The moire removal is effected by a smoothing process using a smoothing filter, one type of the spatial filters. The moire pattern results from interference between periodic density variations in a density pattern of a dotted image and periodic density variations in a density pattern of an output image based on the halftone representation method. Since there are plural types of dots of different scanning densities, the moire-causative spatial frequency varies depending upon the dot types. Where a single image (original) includes plural types of dots, or where each image contains a different type of dot, it is quite difficult for a single spatial filter to exclusively attenuate individual moire-causative spatial frequency components of the image data. If the smoothing filter has such a strong attenuation characteristic for the overall frequency band as to cope with the plural types of dots, the smoothing filter attenuates not only the moire-causative spatial frequency components but also spatial frequency components which should not be involved in the moire removal.
The prior-art technique for removing moire due to the interference of the periodic density variations in density pattern has been disclosed in Japanese Unexamined Patent Publications JP-A 4-104576 (1992) and JP-A7-220072 (1995). An image reader of JP-A 4-104576 includes a low-pass filter with variable cut-off frequencies which is disposed rearwardly of an image-pickup unit in the image reader. The cut-off frequency of the low-pass filter is shifted according to the resolution of the image reader or the type of original to be read by the image reader.
The image processor of JP-A 7-220072 includes a plurality of band-banded cut filters with different cut-off frequencies so that the smoothing filter for removing moire fringes is selectively embodied by combining more than one of the cut filters. The image processor applies 1-D Fourier transform to input image data acquired by reading the original, and uses the results of the Fourier transform to determine a screen angle between a direction of a dot array on the original and an original reading direction, as well as the dot type. Then, the image processor selectively invokes a smoothing filter capable of removing the moire fringes resulting from the type of dots and the screen angle thus determined so as to subject the input image data to the smoothing process using the smoothing filter.
A high-frequency emphasizing filter shown in FIG. 41 may be employed as the edge-emphasizing spatial filter. As shown in FIG. 42, the high-frequency emphasizing filter has a characteristic wherein MTF (modulation transfer function) linearly increases from a predetermined reference value. A graphic representation of a characteristic curve of the high-frequency emphasizing filter is resemblant to that of direct proportion function. Where the high-frequency emphasizing filter is embodied in a 3×3 digital filter, a characteristic of a high-frequency spatial filter is defined by a matrix shown in FIG. 43. In the matrix of FIG. 43, a filter coefficient at the center is of a greater value than the other filter coefficients while difference between the central filter coefficient and the non-central coefficient decreases with increase in the distance between the central filter coefficient and the non-central coefficient.
The edge emphasizing process using the spatial filter is applied only to an image data portion corresponding to an edge. Accordingly, the image processor further includes an edge extraction section for extracting an edge in the image.
The precision of the edge extraction section depends upon an input precision of the image input unit. Where the image input unit has an input resolution of 600 dpi (dot per inch), for example, it is difficult for the edge extraction section to extract a line edge of less than 600 dpi. If this edge extraction section is used to extract an edge of a fine density pattern of high spatial frequencies above a predetermined reference value, no edge is extracted. Where the image contains a density pattern of spatial frequencies below the input resolution, the image input unit detrimentally adds to the image data noise components causing color shift or color turbidity of the image. As a result, it is difficult for the edge extraction section to separate the overall spatial frequency band of the image into a sub-band exclusively containing density-pattern spatial frequencies permitting the edge extraction and a sub-band exclusively containing density-pattern spatial frequencies not permitting the edge extraction, by determining a given spatial frequency as boundary therebetween. Where the image contains a density pattern of spatial frequencies at and about the boundary, the edge extraction section may fail to extract an edge of the density pattern.
Where the image processor performs the edge emphasizing process based on the extraction output from the edge extraction section, the density pattern of the spatial frequencies of the boundary between these sub-bands and its neighborhood may be partly emphasized or unemphasized. This results in density variations of a density-pattern area in the image so that an image outputted based on the image data through the edge emphasizing process is degraded in quality.
Japanese Unexamined Patent Publications JP-A 5-145759 (1993), 5-34435 (1993), 10-28225 (1998) disclose edge emphasizing processes using spatial filters, respectively. The image processor of JP-A 5-145759 is provided with edge-amount sensing means for sensing an amount of edge in an image, an edge emphasizing filter and a mixer for the purpose of edge emphasis. The mixer mixes together image data before and after the processing of the edge emphasizing filter at a mixing ratio based on the edge amount sensed by the edge-amount sensing means.
An apparatus employing the image processing method of JP-A 5-344345 performs the following operations prior to the edge-emphasizing filter process: segmenting a subject image into plural blocks; determining a filter strength for each of the blocks; and correcting the respective filter strengths for the blocks so as to prevent the filter strengths for adjacent two blocks from differing too much each other. After the filter strength correction, the filter of the apparatus using this image processing method filters the respective blocks based on the respective filter strengths thus corrected.
The MTF correction unit of JP-A 10-28225 is provided with the smoothing filter, edge emphasizing filter and filter performing no processing for the purpose of edge emphasis. Prior to the filter process, the MTF correction unit determines each of the all pixels constituting a subject image for the edge sharpness and coarseness, values indicative of the image quality, and then selects any one of the three filters by comparing the edge sharpness or the coarseness with a predetermined reference value. After the filter selection, all the pixels are each subjected to the edge emphasizing process using any selected one of the filters.
As mentioned above, in the image reader of JP-A 4-104576, the moire-removing low-pass filter defines the cutoff frequency according to the reading resolution. The spatial frequencies of dots in the subject image are not constant, varying depending upon the dot types. Therefore, if the cutoff frequency of the smoothing filter is shifted based on the reading resolution alone, the filter accomplishes such a poor moire removal effect, failing to provide a favorable smoothing filter process. Additionally, the low-pass filter attenuates spatial frequency components above the cutoff frequency, thus resulting in the attenuation of not only the moire-causative spatial frequencies but also spatial frequency components necessary for maintaining the output image quality. Hence, the quality degradation of reproduced images results.
In the image processor of JP-A 7-220072, the smoothing filter responsible for the smoothing process for moire removal is embodied in the combination of more than one band-cut filters of narrow bands. This results in the attenuation of only the components of the cutoff frequency of the band-cut filter and of its neighborhood. Accordingly, the moire appearance due to the remainder spatial frequencies is not prevented. Hence, the quality of a processed image is degraded because uniform moire suppression is not established for the entirety of the spatial frequencies of the processed image.
As mentioned above, the image processor of JP-A 5-145759 need to perform the sensing of the edge amount and the image data mixing in addition to the edge-emphasizing filter process. This complicates the operations done by the image processor as a whole and hence, the processor has a complicated configuration. Prior to the filter process, the apparatus using the image processing method of JP-A 5-344345 need to perform the operations of determining the filter strength for each block, detecting a boundary plane, and correcting the filter strength for the boundary plane. Thus, such an apparatus must be involved in complicated operations. The MTF correction unit of JP-A 10-28225 need to perform the calculation of the edge sharpness and coarseness as well as the selection from the three filters. This complicates the operations done by the unit as a whole and hence, the unit has a complicated configuration. In addition, the MTF correction unit selectively switches between the smoothing filter and the edge emphasizing filter based on the reference values and therefore, density variations may occur in a portion consisting of pixels with the values of edge sharpness and coarseness close to the reference values.